A reflective display device such as a liquid crystal cell has to be lit by the front face thereof, i.e. on the side where information is displayed. Thus a lighting system, commonly called a front-lighting system is used. Various embodiments of front-lighting systems are known in the state of the art but, until now, they have found little application in the market. This is due in particular to the fact that it is difficult to make a front-lighting system that has a high level of lighting efficiency without the structures involved making the displayed information difficult to read. Moreover, the yield of currently known front-lighting systems is not very satisfactory in terms of the electric power consumed and the optical power returned and the cost price thereof makes them hardly compatible with the requirements of industrial manufacture.
The system most often envisaged for making a front-lighting device adopts the operating principles of a back-light type lighting device. This type of back-lighting device is shown schematically in FIG. 1, annexed to this patent application. Designated as a whole by the general reference number 1, the back-lighting device includes a plane, transparent light guide 2 arranged underneath a data display device 4 that is to be lit, such as a liquid crystal cell. A light source 6 injects light into guide 2 through one side thereof. Light guide 2 is machined so as to create light extractors 8 therein which deviate part of the light propagating inside guide 2 towards display device 4. Light extractors 8 may be microprisms, micro-lenses, a diffractive array or other. The assembly is completed by a mirror 10 arranged beneath the light guide 2. Of course, the data display device 4 is of the transmissive type.
Application of the back-light system to a reflective type of data display device is illustrated in FIGS. 2A and 2B annexed to this patent application. In this case, a plane, transparent light guide 12 is arranged above a reflective type of data display device 14 which has to be lit, such as a reflective liquid crystal display cell. In photopic vision, sunlight passes through light guide 12 and is reflected by display cell 14 so as to pass through light guide 12 again and reach the observer. In scotopic vision, the light produced by a light source 16 is injected through one side of light guide 12 and is deviated by extractors 18 (microprisms, micro-lenses, diffractive array) in the direction of display cell 14. The light is finally reflected by display cell 14 through light guide 12 towards the observer.
In the case of a system for injecting light into a light guide of the type described above, the first parameter to be optimised is the uniformity of the luminous flux in the entire section of the light guide. This condition is important for guaranteeing uniform illumination of the display device across the entire surface thereof. As shown in FIG. 3, annexed to this patent application, the uniformity of illumination of display device 14 in the direction of the length L of light guide 12 is determined by the distribution of extractors 18 over the surface of guide 12. As regards the uniformity of illumination of display device 14 in the direction of width I of light guide 12, this depends upon the geometry of light guide 16.
Thus, in the case of devices which are not subject to constraints in terms of electric power consumption such as, for example, liquid crystal computer screens, the typical geometry of light source 16 is that of a fluorescent tube known as a cold cathode fluorescent lamp or CCFL, the length of which substantially corresponds to the width of the liquid crystal display device. However, although these fluorescent tubes are becoming increasingly efficient from the point of view of yield, the electric power consumption thereof still remains incompatible with use in portable electronic devices such as, for example, watches, whose energy reserves are necessarily limited.
Consequently, the light sources most commonly used in these portable objects such as wristwatches are light emitting diodes or LEDs. Light emitting diodes are characterized notably by a high level of luminance insofar as their entire luminous energy is emitted from a small surface and they have a higher conversion yield than all conventional light sources as regards the electric power consumed and the optical power provided. However, it is difficult to illuminate a display device in a uniform manner using a light emitting diode due to the reduced dimensions of a diode of this type.
FIGS. 4A and 4B annexed to this patent application illustrate two solutions commonly used in current devices for injecting the light produced by a light emitting diode. In the first case (FIG. 4A), the light emitting diode 20 is placed directly opposite the input of light guide 22. This configuration produces a high injection yield for the light produced by light emitting diode 20. However, because of the luminous emitting lobe typical of a light emitting diode which is of the Lambertian type, the light inside guide 22 propagates in a large number of different directions. The light extraction efficiency of extractors 24 essentially depends on their orientation relative to the light propagation direction. Thus, both in the design phase and in the manufacturing phase of light guide 22, the orientation of each extractor 24 must be determined and controlled relative to the position of light emitting diode 20. This becomes a complex task if light extractors 24 are microprisms, and almost impossible to resolve if it is desired to use a diffractive array. Indeed, in this latter case, a diffractive array with a curved line profile would have to be developed. Moreover, with this injection system, side lighting is not uniform given that because of the lobe-shaped emission profile of the light emitting diode, the light injection yield is higher at the small injection angles than at the large injection angles.
In the second case illustrated in FIG. 4B, the light is first injected into a reflective/refractive structure 26, which has the capacity to reflect light into guide 28 in substantially parallel propagation directions. With this type of arrangement, most of the light rays propagate parallel to the axis of light guide 28. However, for light injection into guide 28 to be homogeneous, deflector 26 must be designed to be able to inject a constant quantity of light over the entire width of guide 28. This arrangement is preferably used when good quality lighting (collimation) is desired for the display device rather than intense lighting. In fact, the efficiency of the injection coupling of light into the guide is generally less than one third of the initial optical power of the light source.
From the foregoing it is clear that, depending on the solution retained for injecting light produced by a light source into a light guide, priority is given either to the efficiency of light injection into the guide and thus the quantity of light which will eventually light the display device, or the uniformity of the light injected into the guide and thus the lighting quality of the display device. To the Applicant's knowledge, there does not exist a lighting device which lights a display device in both a luminous and uniform manner.
To respond to this need, in addition to others, in the state of the art, the present invention provides a lighting system for a data display device that guarantees excellent coupling efficiency of the light injection into the light guide according to the invention as well as a high collimation level of the luminous flux propagating in the light guide.